Iron garnet R.sub.3-x Bi.sub.x (Fe, M).sub.5 O.sub.12 obtained by substituting Bi for R in a rare-earth iron garnet R.sub.3 (Fe, M).sub.5 O.sub.12 is recently receiving much attention as a magneto-optic recording material. Note that in the above formulae, R is a rare-earth element, and M is an atom having a valence of 3 or a group of atoms having a valence equivalent to a valence of 3 such as Al.sup.3+, Ga.sup.3+, Sc.sup.3+, Tl.sup.3+, (Co.sup.2+ +Ti.sup.4+) or the like. In the Bi-substituted rare-earth iron garnet, since some Rs are substituted with Bi, the Faraday rotation angle .theta..sub.F can be increased without much increasing the absorption coefficient .alpha. and, therefore, this material is generally an excellent magneto-optic recording material.
In order to improve performance of a magneto-optic material of the Bi-substituted rare-earth iron garnet having such a property, the Bi substitution amount x must be increased, which results in the increase of the Faraday rotation angle .theta..sub.F. Conventionally, in a sintered ceramics of a rare-earth iron garnet, the Bi solid solution limit is known to be 50% of the dodecahedral site of the crystal structure. So monocrystalline films having a large Bi substitution amount x have been studied intensively.
A highly Bi-substituted rare-earth iron garnet thin film as described above is normally manufactured by the liquid phase epitaxy (LPE) method as described, e.g., in J.J. A. P., 19, 2105 (1980). However, a magnetic thin film manufactured by the liquid phase epitaxy method, wherein the film composition is Bi.sub.0.6 Sm.sub.1.2 Er.sub.1.2 Ga.sub.1.0 Fe.sub.4.0 O.sub.12 has a small Faraday rotation angle .theta..sub.F of about 2.4 degree/.mu.m at a light wavelength .lambda.=500 nm at which the Faraday rotation angle is expected to be maximum value. Such a small Faraday rotation angle is impractical.
Further, in LPE method, a Bi-substituted rare-earth iron garnet film must be formed on a single crystal substrate such as gadolinium gallium garnet Gd.sub.3 Ga.sub.5 O.sub.12 (to be referred to as a GGG substrate hereafter) which is high is cost. However, a method capable of forming a highly Bi-substituted rare-earth iron garnet thin film on an amorphous substrate such as a glass substrate which has a high productivity has been desired. Although various studies have been made to provide such a method, a Bi-substituted rare-earth iron garnet thin film formed on an amorphous substrate obtained so far has been a polycrystalline film having a magnetization parallel to the film plane. A vertically magnetized film desired as a magneto-optic recording material has not yet been obtained.